The invention relates to a photovoltaic cell comprising at least one electrically conductive back contact, a first semiconductor layer arranged on said contact, an adjacent second semiconductor layer of different conductivity type or a Schottky barrier layer, and an electrically conductive front contact, in particular to a copper sulphide/cadmium sulphide thin film solar cell, and to a method for fabrication of a said cell.
In photovoltaics, there is a choice of expensive crystalline solar cells with an efficiency of 10% and above, and cheap thin film solar cells with an efficiency of 4 to 8%, of the amorphous silicon or copper sulphide/cadmium sulphide (Cu.sub.2 S/CdS) types. The former are not in demand due to their high manufacturing costs; but the less expensive thin film solar cells are also unable to achieve the hoped-for breakthrough, particularly because they are insufficiently stable over a long period, for example 6 to 10 years, i.e. they fail to retain their initial power. The efficiency of a thin film solar cell of the Cu.sub.2 S/CdS type is strongly dependent on the stoichiometry of the copper sulphide, which has the function of the absorber in the solar cell, so that a direct influence can be derived on the number of charge carriers formed and on the current density achievable. To show the optimum characteristics, the stoichiometry should be between Cu.sub.1.995 S and Cu.sub.2.000 S. Since this copper sulphide represents the surface of the solar cell system, interaction with the oxygen in the air or the cover materials used must be considered. Copper oxide Cu.sub.2-z O (with 0.ltoreq.z.ltoreq.1) is formed, drawing its copper atoms from the copper sulphide, which thus has the function of the copper supplier for the copper oxide. Since the copper sulphide layer is only approx. 300 nm thick and the growing copper oxide can increase up to a thickness of 5-10% of the thickness of the copper sulphide, a considerable reduction of the copper stoichiometry can be observed as a consequence; the stoichiometry drops to Cu.sub.1.90 S even when the initial values are very good.
DE-OS No. 21 52 895 attempts to increase the efficiency of a thin film solar cell of the copper sulphide/cadmium sulphide type and to maintain it at a high level by the copper oxide layer, resulting from degradation, being formed simultaneously with an improvement in the stoichiometry of the copper sulphide. Ageing of the front copper sulphide semiconductor would therefore be incorporated into the manufacturing process of the solar cell. To do so, it is proposed that a further copper-containing layer be deposited onto the copper sulphide layer, said copper-containing layer being thin in relation to the copper sulphide layer. The additional layer can be obtained by, for example, deposition of an additional layer containing copper, and subsequent heat treatment in vacuum or in air. It is also possible to generate the copper content for the additional layer by reducing the copper sulphide layer on the surface. To do so, the surface can be reduced by treatment in a glow discharge in a hydrogen atmosphere. However, all these measures to provide for arrangement of a separate layer onto the copper sulphide layer, have not yet shown the hoped-for success as regards longterm stability.
The object of the present invention is to develop a photovoltaic cell of the type mentioned at the outset in such a way that the front layer, i.e. the second semiconductor layer, is formed as a whole with high stoichiometry, i.e. without vacancies, and that the layer so formed is not subject to copper, thus stoichiometric, depletion even over long periods, so precluding the possibility of degradation. (High stocihiometry is used in this instance in the semiconductor technology sense and means that each sulphur ion is matched with exactly 2 copper ions. This absence of vacancies is expressed by Cu.sub.2.0000 S for the copper sulphide. Formulations like, for example, Cu.sub.2.00 S or Cu.sub.2 S corresponding to chemical nomenclature, do not adequately express the desired absence of vacancies.)
The object is attained in accordance with the invention by the formation of an inversion layer on the semiconductor and which is tunnelled through by electrons or holes but not by ions, and that a low-resistivity layer that conducts electrons or holes well is arranged between the semiconductor inversion layer and the electrically conductive contact.
The formation of the inversion layer on the surface of the second semiconductor layer ensures that a stoichiometry once given, is not deteriorated, so preventing a reduction of the curent density achievable during irradiation. This is achieved by the function of the inversion layer, which interdicts penetration by ions, but permits the passage of electrons or holes, which are the charge carriers determining the conversion efficiency of the solar cell (the probability of ion tunnelling is negligible in view of the considerably greater mass of the ions compared with the mass of the electrons or holes).
A particular feature of the invention is a photovoltaic cell in the form of a Cu.sub.2-x S/CdS thin film solar cell with 0.ltoreq.x.ltoreq.1. The Cu.sub.2-x S layer is formed with high stoichiometry having approximately x=0.0000 and the inversion layer is arranged on the surface of the Cu.sub.2.0000 S layer. The inversion layer can have a thickness from approx. 1 to 5 nm.
The inversion layer is generated by strongly reducing species, for example a negatively charged hydrogen atom (H.sup.-) interacting with the surface of the p-semiconductor, i.e. the copper sulphide.
These reducing adsorbates act on the surface layer as electron donators. The result is that the surface layer receives a negative potential while the layer beneath it receives a positive one for charge neutrality reasons. For ions, these layers are an impenetrable potential wall, while the charge carriers (electrons and holes) can tunnel through them. The copper sulphide surface built up as an inversion layer therefore acts as a semipermeable wall.
The precondition for formation of a suitable inversion layer is that the copper sulphide is pure copper sulphide, with neither copper oxide nor excess sulphur ions being present. Since these components disturbing the stoichiometry of the copper sulphide layer always crop up during fabrication of the said layer, the invention selectively reduces copper oxide on the surface of the copper sulphide layer and removes free sulphur ions, and then by adsorption of the reducing species and interaction thereof with the p-semiconductor Cu.sub.2.0000 S, converts the surface of the said semiconductor into an inversion layer. If, for example, there was still an oxide layer on the p-semiconductor at this juncture, formation of an inversion layer would not be possible. The reason for this is that when electron holes are present in the not highly stoichiometric Cu.sub.2-x S or Cu.sub.2-z O, the electrons of the adsorbed donators directly recombine with these electron holes, as is required in DE-OS No. 21 52 895 (wherein the surface layer of the copper sulphide is converted to copper without the volume material beneath it being changed). The inversion layer ensures that there is no degradation of the copper sulphide layer even over long periods, i.e. that a high stoichiometry is retained once it has been achieved.
Since, however, the pure copper sulphide layer is of relatively high electrical resistivity, so that movement by the charge carriers to the electrically conductive front contact, determining the conversion efficiency of the solar cell, is impeded, a further embodiment of the invention involves continuation of the reduction process even after the build-up of the inversion layer, in order to convert the free surface thereof into pure copper which is then selectively oxidized to form Cu.sub.2-y O in a subsequent heating process. This oxide layer is at least one order of magnitude less resistive than Cu.sub.2.0000 S and represents a highly conductive connection for the charge carriers between the front semiconductor layer and the electrically conductive front contact arranged on the oxide layer. (It would theoretically be possible to dispense with the Cu.sub.2-y O layer, provided a dense front contact grid was applied directly to the inversion layer; but this would have the drawback that an unwelcome shading effect would occur. However, degradation would not occur. It is therefore possible to apply the copper layer for the copper oxide in another process too, for example vacuum deposition or galvanic deposition.)
To carry out the reduction in a controlled manner, the known discharge process is modified such that after the electrically conductive rear contact, the cadmium sulphide layer, and the Cu.sub.2-x S layer have been applied, they are placed in a hydrogen atmosphere as a single unit, preferably in the positive column of the glow discharge. Preferably, the carrier with the layers arranged thereon is placed in an initially evacuated space (approx. 10.sup.-6 mbar (10.sup.-4 Pa)), which is then filled with hydrogen to a pressure of approx. 0.5 mbar (50 Pa). Once the pressure has stabilized at 0.5 mbars (50 Pa), the glow discharge proper takes place, in which the hydrogen radicals or H.sup.- ions effect the reduction of the Cu.sub.2-x S layer to an extent that the highly stoichiometric Cu.sub.2.0000 S layer is formed, the surface of which is converted to the inversion layer whose surface is in turn converted to pure copper. It must be ensured here that the reducing species does not merely effect the reduction of the Cu.sub.2-x S layer surface to elementary copper while leaving the volume material located thereunder unchanged as a Cu.sub.2-x S layer, as described in DE-OS No. 21 52 895.
Heating then follows. This can take place, for example, either at 180.degree. C. or 200.degree. C. in vacuum followed by 150.degree. C. in air. By reduction of the Cu.sub.2.0000 S layer after formation of the inversion layer followed by heating, a controlled oxide formation takes place. By way of comparison, a defined thickness of the copper oxide Cu.sub.2-y O is not possible in other manufacturing processes if the copper layer necessary for the oxide layer is directly deposited on the front semiconductor. Here, oxide from the heating stage and residual oxides oxides already formed during the manufacturing process of the semiconductor become mixed.
Furthermore, it must be emphasized that the process steps
(a) Formation of the highly stoichiometric Cu.sub.2.0000 S layer, PA0 (b) Formation of the inversion layer and, if applicable, and PA0 (c) Formation of copper and the copper oxide layer,
follow one another in controlled sequence by recording the H.sub.2 S partial pressure detected in the glow discharge chamber. It is therefore necessary, for example with a current density of 10 .mu.A/cm.sup.2 of semiconductor layer at a voltage between cathode and anode of 300 V and a gas pressure of approx. 50 Pa (5.10.sup.-4 bars) in the glow discharge chamber, that the partial pressure of the H.sub.2 S be steeply rising at the start of the glow process, for a period of approx. 5 mins. (conversion of the Cu.sub.2-x S layer into a Cu.sub.2 S layer), then to remain almost unchanged until approx. 10 mins. after the start of the process (formation of the inversion layer), and then to rise gently (formation of the copper layer).